Air-conditioning can be a considerable contributor to the operational CO2 emissions of a building. Thermal mass can minimise or even eliminate the demand for air cooling, which in turn reduces or eliminates CO2 emissions. New technical design guidelines from the Concrete Centre shows how.
The use of air conditioning is on the rise as the summers become hotter owing to global warming. In the UK the uptake of air-conditioning is rising by 8 percent every year. By 2020, an additional six million metric tonnes of CO2 emissions could be generated. As a countermeasure, the 2006 edition of Part L2A of the Building Regulations establishes strict standards for CO2 emissions, making the installation of a full air-conditioning system tougher to justify, especially in the face of rising energy expenses.
To prepare for the effects of climate change, building codes now incorporate specific overheating limitations for non-air-conditioned structures. Against this background, passive cooling solutions are being widely employed in non-residential buildings.
The building fabric’s thermal mass and the way it interacts with the internal and external environments are key to low-energy design. Concrete’s unrivalled thermal mass plays a key part in maintaining a cosy environment inside. The use of exposed concrete to offer passive cooling can produce significant savings in terms of capital and operational costs through avoiding or minimising the requirement for air conditioning.
The basic method is to expose the soffit of the floor slabs, which can thus absorb heat gains during warm weather and maintain the internal temperature. In order to prepare the building for the following day, night air is typically used to ventilate and cool it. This cycle of heating and cooling using the thermal mass of a building is typically referred to as Fabric Energy Storage (FES) (FES).
A typical office can use FES to put off the peak internal temperature for six hours after the last occupants have left. At this stage the FES cycle is reversed. As the day draws to a close, the outside temperature lowers, making night ventilation an efficient way to prepare the fabric of the structure for the next day by eliminating the accumulated heat. The UK fluctuation in diurnal temperature rarely dips below 5°C making night cooling very effective. Water cooling can be utilised instead of or in addition to night ventilation. Slab cooling may be more adaptable and customizable as a result. There are a lot of general FES systems. These include natural ventilation with exposed soffits, which are flat or contoured floor slabs used in conjunction with natural ventilation. This may be wind-driven, or a mix of wind and stack ventilation.
It is also possible to use underfloor ventilation with exposed soffits for heating and cooling purposes in buildings. The gap generated by a raise floor is employed as a plenum for mechanical ventilation enhancing exposure of the thermal mass by permitting heat transfer with top of the slab in addition to the soffit on the bottom. Air enters the occupied space using floor diffusers. This method is commonly used in conjunction with an exposed, profiled slab and with openable windows to give a mixed-mode solution.
Precast hollowcore concrete slabs with mechanical ventilation via the cores, can provide good convective heat transmission between the air and concrete. Further heat transfer is offered by the exposed slab soffit. The trade term “Termodeck” is commonly used to refer to the system.
In conjunction with a night ventilation strategy, precast or cast in-situ slabs with water cooling provided by embedded polybutylene piping can be used. The precast option is trademarked as “Thermocast”.
Soffits that are partially or completely exposed can be used with chilled beams. These take the form of flat or coffered concrete soffits with chilled beams dangling directly below. The soffit might be uncovered or a permeable ceiling can be installed. FES is delivered in the conventional fashion, using natural and/or mechanical ventilation, with added cooling from the chilled beams.
The use of FES design in conjunction with mixed-mode ventilation and/or air conditioning is common in new construction. This is owing to the necessity for increased cooling performance for demanding office environments and in order to solve issues of security and occupant control of windows.
A high thermal mass building will create a high quality, well-ventilated area in which occupants are empowered to take control of their surroundings. The building’s owner will save money, while the environment will gain from a smaller carbon footprint.
More and more evidence suggests that FES is being employed in the construction of speculative office buildings, and there are numerous examples of this. This is being pushed not only by awareness of the need for greater sustainability, but also by developers and building occupiers seeing the long-term financial benefits of high thermal mass structures. This could lead to FES becoming the standard for commercial construction in the twenty-first century.